TYPES OF SILK YARN- BOMBYX MORI:-

Bombyx Mori:-

The domestic silk moth is an insect from the moth family Bomby cidae. It is the closest relative of Bombayx mandarina, the wild silk and moths. The silk worm is the larvae or catterpillar of a silk moth. It is an economically important insect being a primary producer of a silk. A silk worms preferred food are white mulbery leaves , though they may eat other mulberry species and even the osage orange. Domestic silk moths are entirely dependent on humans for reproduction as a result of millennia , of selective breeding. Wild silk moths (other species of Bombayx) are not as commercially viable in the production of silk.

image of paired moth male (above) female (below) 

Process of sericulture (Bombayx Mori):-

image of Adult silk moth:-

Eggs take about 14 days to hetch in to larvae which eat continuously. They have a preference for white mulberry, having an attraction to the mulberry ordorant cisjasmone. They are not monophagus, since they can eat other species of Morus. as well as some other Moracae , mostly Osage orange. They are covered with tiny black hairs, when the color of their head turns darker , it indicates that they are about to molt. After molting, the larval phase of the silk worm emerge white, naked and with little horn on their back.

After they have molted four times their bodies becomes slightly yellow , and the skin becomes lighter . The larvae then prepare to enter the pupal phase of their life cycle, and enclose  themselves in a cocoon made up of raw silk produced by the salivary glands. The final molt form larvae to pupa takes place within the cocoon , which provides a vital layer of protection during the valunerable , almost motion less pupal state . Many other lepidoptera produce cocoon, but only a few Bombycidae, in particular the genus Bombyx , and the saturniidar, in particular the genus Antheraracea-have been exploited for fabric production.

If the animal is allowed to survive after Spinning its cocoon and through the pupal phase of it life cycle , it release proteolytic enzymes to make a hole in the cocoon, so it can emerge as an adult moth. These enzymes are destructive to the silk and can cause  the silk fibers to break down from over mile of length to segments of random length which seriously reduces the value of silk threads although this damage silk cocoons are still used as "stuffing" available in China and else where for doonas, jackets , etc . To prevent this silk worms cocoon are boiled . The heat kills the silk worms and the water makes the cocoons easier to unravel. Often the silk worm itself is eaten.

As the process of harvesting the silk from the cocoon kills the larvae, sericulture has been criticised by animal welfare and right activists "Mhatma Gandhi" was critical of silk production based on the Ahimsa philosophy "not to hurt any living thing," This led to Gandhi promotion of cotton Spinning machine an example of which can be seen at Gandhi Institute and an extension of the principle has led to modern production practice known as Ahimsa silk which some  is wild silk (from wild and semi wild silk moth) made from the cocoons of moths that are allowed to emerge before the silk is harvested. 

The moth- the adult phase of the life cycle is not capable of functional flight , in contrast to the wild B. mandarina and other Bombyx species whose make fly to meet female and for evasion from predators. Some may emerge with the ability to lift off and stay airborne , but sustained flight cannot be achieved. This is because their bodies are too bih and heavy for the small wings. Silk moth have a wing span of 3-5 cm (1.2-2.0 in) and a white hairy body . Females are about two to three times bulkier than males (for they are carrying many eggs) but are similarly  colored. Adults Bombycidae have reduce moth parts and do not feed.

COCOON:-

image of cocoon B. Mori.

The cocoon is made of thread of a raw silk from 300 to about 900 m (1,000 to 3000 ft) long. The fibers are very fine and lustrous, about 10 um (0.0004 in) in diameter.  About 2,000 to 3,000 cocoon are required to make one pound of silk (0.454 kg) . Atleast 70 millions pounds (32 million kgs) of raw silk are produced each year, requiring nearly 10 billions cocoons.

Ref:-https://en.m.wikipedia.org